Euclid is a space mission of the European Space Agency (ESA) that will map the geometry of the dark Universe and the cosmic history of structure formation in the Universe by taking images and spectra of thousands of millions of galaxies. The mission, to be launched in 2020, will provide a wealth of unprecedented high quality data collected with two different instruments: an imager at visible wavelengths (VIS) and an spectro-photometer in the near infrared (NISP) . VIS and NISP are built by the Euclid Consortium (EC), an organisation that brings together international teams of astronomers and physicists in charge of the production of the data and the scientific exploitation of the Euclid mission. The combination of these two powerful instruments will provide a unique window to the early stages of evolution of the universe. In particular, it will shed light on the nature of the mysterious dark-energy that drives the observed accelerated expansion of theuniverse, and test Einstein’s general relativity theory on the largest cosmological scales.

Mining this big and complex cosmological dataset is a formidable challenging task involving ESA and hundreds of scientists of the Euclid Consortium from 14 European countries (Austria, Belgium, Denmark, Finland, France, Germany, Italy, the Netherlands, Norway, Portugal, Romania, Spain, Switzerland and United Kingdom), the United States and Canada. A key ingredient in order to prepare the scientific exploitation of this “golden” dataset is the development of synthetic observations of the real survey: a cosmological simulation that matches the expected volume and complexity of the real data. In a massive coordinated effort, a team of scientists of the Euclid project have worked together over the last year to develop the largest simulated galaxy catalogue ever produced, the so-called “Euclid Flagship” mock galaxy catalogue

The Euclid Flagship mock galaxy catalogue is based on the record-setting 2 trillion (2 followed by 12 zeros) dark-matter particle simulation performed on the supercomputer Piz Daint, hosted by the Swiss National Supercomputing Center (CSCS). The simulation code was developed by a team of scientists at the University of Zurich, led by Joachim Staled from the Institute for Computational Science. This unique dataset reproduces with exquisite precision the emergence of the large scale structure of the Universe, with hundred of billions of dark matter halos hosting the galaxies we see in the night sky today.

Using this dark-matter cosmic web from the Flagship simulation, a group of scientists of the Euclid Consortium at Institut de Ciències de L’Espai (ICE, IEEC-CSIC) and Port d’Informació Científica (PIC) in Barcelona, in collaboration with the Cosmological Simulations Working Group, led by Pablo Fosalba (ICE, IEEC-CSIC) and Romain Teyssier (Institute for Computational Science at the University of Zurich), have built a synthetic galaxy catalogue using state-of-the-art scientific pipelines that implement the Halo Occupation Distribution technique, a sophisticated recipe to relate dark and luminous matter in the universe.

The Euclid Flagship mock galaxy catalogue contains more than 2 thousand million galaxies distributed over the 3-dimension cosmological volume that Euclid will survey. Synthetic galaxies in this simulation mimic with great detail the complex properties that real sources display: ranging from their shapes, colours, luminosities, and emission lines in their spectra, to the gravitational lensing distortions that affect the light emitted by distant galaxies as it travels to us, the observers. A dedicated web portal, CosmoHub hosted by PIC, the Euclid Spanish Science Data Center, will distribute the Flagship mock data to the 1000+ members of the Euclid Consortium.

Armed with this new virtual universe, scientists will be able to assess the performance of the Euclid mission as a whole, the so-called Science Performance Verification. The Science Performance Verification exercise uses a full end to end simulation of the Euclid mission developed by ESA and the Euclid Consortium and represents a critical milestone of the project. Moreover the Euclid Flagship mock will be an essential tool to develop the data processing and the science analysis pipelines developed by the Euclid Science ground Segment and the Science Working Groups and will set the science for the exciting discoveries that await the Euclid mission when the real data shall come.

Euclid Flagship mock galaxy catalogue – False colour images showing a small portion (0.3%) of the full light-cone simulation of mock galaxies in the Euclid survey. Light-cone stripes extend 500 Mpc/h (vertical) x 3800 Mpc/h (horizontal axis). The 2-dimension “pencil beam” images result from a slice of the 3-dimension light-cone, projected from a 40 Mpc/h width (in the direction orthogonal to the image plane). From top to bottom, panels display the full sample of galaxies in the mock, and the sub-samples expected from observations in the VIS and NISP-Halpha channels. The galaxy mock has been produced using a Halo Occupation Distribution pipeline developed by the Institut de Ciències de l’Espai (ICE) and Port d’Informació Científica (PIC) in Barcelona, and it is based on the 2 trillion dark-matter particle Flagship run produced by U.Zurich.

Galaxy Types in the Flagship Mock Catalogue – Top panel: False colour images showing a small portion (0.3%) of the full light-cone simulation (similar to Image 1), but showing different galaxy types with different colours. Central galaxies are coloured in green, and satellites in red. Bottom panel: zoom in of the top panel image that displays the local universe with greater detail. Central galaxies populate all dark-matter halos of the cosmic web, whereas satellite galaxies tend to reside in the most massive halos, that is, in the highest density peaks of the underlying dark-matter distribution.